In the field of microwave energy, it is well known that microwave ovens, furnaces, and the like are typically constructed with a fixed operating frequency. It has long been known that the interactions of various materials with microwaves are frequency dependent. These interactions may include curing rubber and sintering ceramics. It is therefore desirable to have a microwave furnace that can be operated over a broad frequency range.
Typical microwave energy sources have a very narrow bandwidth because they employ a resonant cavity. For example, microwave ovens constructed for home use are provided with a magnetron which operates at 2.45 GHz, which is a frequency that has been allocated by the FCC for domestic heating applications. Due to the coupling ability of a 2.45 GHz microwave to water, these ovens are used for cooking foods, drying, and other purposes wherein the principal material to be acted upon is water. However, it is well known that frequencies in this range are not optimal in all situations, such as with heating plasmas, sintering materials such as ceramics, and preparing films such as diamond films.
The use of frequency sweeping over a wide range as a means of mode stirring has important implications for the use of microwave power to sterilize medical equipment or contaminated wastes. In such uses it is crucial to eliminate "dead" areas in the cavity wherein sufficient power may not be received in order for complete sterilization. Electronic frequency sweeping may be performed at a high rate of speed, thereby creating a much more uniform time-averaged power density throughout the furnace cavity. The desired frequency sweeping may be accomplished through the use of a variety of microwave electron devices. A helix traveling wave tube (TWT), for example, allows the sweeping to cover a broad bandwidth (e.g., 2 to 8 GHz) compared to devices such as the voltage tunable magnetron (2.45.+-.0.05 GHz). Other devices such as klystrons and gyrotrons have other characteristic bandwidths, which may be suitable for some applications.
Further, fixed-frequency microwave ovens typically found in the home are known to have cold spots and hot spots. Such phenomena are attributed to the ratio of the wavelength to the size of the microwave cavity. With a relatively low frequency microwave introduced into a small cavity, standing waves occur and thus the microwave power does not uniformly fill all of the space within the cavity, and the unaffected regions are not heated. In the extreme case, the oven cavity becomes practically a "single-mode" cavity.
Attempts have been made at mode stirring, or randomly deflecting the microwave "beam", in order to break up the standing modes and thereby fill the cavity with the microwave energy. One such attempt is the addition of rotating fan blades at the beam entrance of the cavity.
Another method used to overcome the adverse effects of standing waves is to intentionally create a standing wave within a single-mode cavity such that the workpiece may be placed at the location determined to have the highest power (the hot spot). Thus, only that portion of the cavity wherein the standing wave is most concentrated will be used.
The frequency for most efficient processing may vary for a given material as the heating process occurs. As a material changes phases, a varied frequency may be required. Thus, it may be desired to have the capability of varying the frequency in the heating process, allowing the operator to begin heating the workpiece at one frequency and then change the frequency to maintain good coupling as the temperature rises. This may also be desirable when heating composite materials, where the varying materials efficiently react at different frequencies.
Other devices have been produced to change the parameters of the heating process of selected materials. Typical of the art are those devices disclosed in the following U.S. Patents:
Patent No. Inventor(s) Issue Date 3,611,135 D. L. Margerum October 5, 1971 4,144,468 G. Mourier March 13, 1979 4,196,332 A. MacKay B, et al. April 1, 1980 4,340,796 M. Yamaguchi, et al. July 20, 1982 4,415,789 T. Nobue, et al. November 15, 1983 4,504,718 H. Okatsuka, et al. March 12, 1985 4,593,167 O. K. Nilssen June 3, 1986 4,777,336 J. Asmussen October 11, 1988 4,825,028 P. H. Smith April 25, 1988 4,843,202 P. H. Smith, et al. June 27, 1989 4,866,344 R. I. Ross, et al. September 13, 1989 4,939,331 B. Berggren, et al. July 3, 1990 5,321,222 D. W Bible et al. June 14, 1994
The subject matter disclosed by MacKay ('332) is further discussed in an article authored by MacKay B, et al., entitled "Frequency Agile Sources for Microwave Ovens", Journal of Microwave Power, 14(1), 1979.
A microwave furnace having a wide frequency range is described in commonly assigned U.S. Pat. No. 5,321,222, the entire disclosure of which is incorporated herein by reference.
Pending commonly assigned application Ser. No. 08/413,608, filed on Mar. 30, 1995, now U.S. Pat. No. 5,961,871, the entire disclosure of which is incorporated herein by reference, describes how frequency sweeping over a selected bandwidth, typically 5%, can establish a substantially uniform microwave power distribution within the cavity by the superposition of many hundreds of microwave modes.